1. Field of the Invention
The invention relates to a brake controller, brake control system and brake control method that control a braking force applied to drive wheels provided for a vehicle.
2. Description of the Related Art
In recent years, an electronically controlled brake system is widely employed as a braking system for a vehicle (see Japanese Patent Application Publication No. 2006-27453 (JP-A-2006-27353)). The electronically controlled brake system controls a braking force applied to each drive wheel so as to apply an optimal braking force to the vehicle in accordance with a running condition of the vehicle.
Such an electronically controlled brake system includes pressure-increasing linear control valves and pressure-reducing linear control valves in passages of brake fluid. A hydraulic pressure difference between the upstream side and downstream side of each of these pressure-increasing linear control valves and pressure-reducing linear control valves is linearly controlled with respect to an electric current supplied. Each of the linear control valves is a linear valve that is able to selectively control a wheel cylinder pressure, which is a hydraulic pressure applied to a wheel cylinder. Each of the linear control valves is able to continuously vary the wheel cylinder pressure by changing an electric current supplied thereto.
Each pressure-increasing linear control valve is provided between a power hydraulic pressure source and the corresponding wheel cylinder. Each pressure-increasing linear control valve controls the hydraulic pressure of the power hydraulic pressure source and supplies the hydraulic pressure to the corresponding wheel cylinder to increase the wheel cylinder pressure. Each pressure-reducing linear control valve is provided between a reservoir and the corresponding wheel cylinder. Each pressure-reducing linear control valve controls a drain of brake fluid from the corresponding wheel cylinder to the reservoir to decrease the wheel cylinder pressure.
In the electronically controlled brake system, as a brake pedal is depressed, a target hydraulic pressure of each wheel cylinder is set on the basis of a master cylinder pressure generated by the depression, electric currents supplied to the pressure-increasing linear control valves or the pressure-reducing linear control valves are determined so as to obtain the target hydraulic pressure applied to each wheel cylinder, and then the pressure-increasing linear control valves or the pressure-reducing linear control valves are opened to increase or decrease the wheel cylinder pressures. Then, as the depression of the brake pedal is released, electric currents are supplied to the pressure-reducing linear control valves. Then, the pressure-reducing linear control valves are opened for a predetermined period of time to drain the brake fluid in the wheel cylinders to the reservoir. Thus, the remaining pressures become zero.
Incidentally, in the electronically controlled brake system, electric currents supplied to the above described pressure-increasing linear control valves and pressure-reducing linear control valves are normally controlled through pulse width modulation (PWM) control by an electronic control unit (ECU). However, this PWM control generates high-frequency switching noise. The noise is hardly audible to an occupant of a vehicle because road noise is large when the vehicle is running. On the other hand, when the vehicle is stopped or running at an extremely low speed, road noise is small, so the high-frequency noise may be audible to the occupant. Particularly, in the above described control executed to eliminate the remaining pressures after the depression of the brake pedal is released, there is a possibility that the occupant feels uncomfortable against noise generated when the pressure-reducing linear control valves are opened.